Sesquioxides vaporization

It might be expected that lanthanides which exhibit divalent states, like Eu and Yb, would be most likely to have their sesquioxides vaporize via the generation of monoxide vapor species. Instead these sesquioxides tend to form atomic vapor (e.g., R) species and oxygen due to the greater stability of R versus RO. Thus, for the sesquioxides of actinides with a greater tendency towards divalency or that are divalent metals (e.g., Cf, Es and Fm), the more likely it is that the vaporization mode of their sesquioxides is via generation of elemental vapor plus oxygen. 

It has been generally assumed that the rare-earth sesquioxides vaporize con-gruently with almost stoichiometric composition though a few studies (White et al. 1962, Ackermann and Rauh 1971, 1975) have shown small deviations (R2O2 99 to R2O2 995) from ideal stoichiometry. But this approximation had insignificant contribution to the reliability of the thermodynamic data of monoxides. 

In an alternative industrial process, resorcinol [108-46-3] is autoclaved with ammonia for 2—6 h at 200—230°C under a pressurized nitrogen atmosphere, 2.2—3.5 MPa (22—35 atm). Diammonium phosphate, ammonium molybdate, ammonium sulfite, or arsenic pentoxide maybe used as a catalyst to give yields of 60—94% with 85—90% selectivity for 3-aminophenol (67,68). A vapor-phase system operating at 320°C using a siUcon dioxide catalyst impregnated with gallium sesquioxide gives a 26—31% conversion of resorcinol with a 96—99% selectivity for 3-aminophenol (69). 

This paper reviews data on certain thermodynamic aspects of the nonstoichiometric Pu-0 system, which may serve as a basis for use In reactor safety analysis. Emphasis Is placed on phase relationships, vaporization behavior, oxygen-potential measurements, and evaluation of pertinent thermodynamic quantities. Limited high temperature oxygen potential data obtained above the fluorite, diphasic, and sesquioxide phases In the Pu-0 system are presented. 

Two equivalents of protosulphate of iron are decomposed into one equivalent of sesquioxide, one of sulphuric acid, and one of sulphurous acid one of the equivalents of sulphuric acid has parted with one of its equivalents of-oxygen, in order that the two equivalents of protoxide of iron might become sesquioxide. This equivalent of sulphurous acid is said to escape through the joints of the apparatus if so, and there does not appear to be any doubt of this boing the fact, inasmuch, as no other provision appears to have been made for its exit, one may naturally oonclude that sulphuric acid vapor must escape at the same time. 

A mixture of sulfur dioxide. S02. and sulfur vapor, at low pressure and with an electric discharge, forms sulfnr monoxide, SO Its presence is shown from its absorption spectrum, but upon separation it dispropor-tionates at once to sulfur and S02. Sulfur sesquioxide. S2Ojt, is formed by reaction of powdered sulfur with anhydrous SO3 S20 also dispropor-tionates (at 20°C in nitrogen) to sulfur and S02, Sulfur dioxide, S02, is... 

According to Ames et al. (1967) the vaporization of rare earth sesquioxides can be represented by the equation... 

Vaporization characteristics of rare earth metals and rare earth sesquioxides. 

Thin films of the oxides can be obtained by oxidation of evaporated or sputtered metallic films under suitable temperatures and oxygen pressures. The sesquioxides are the congruently vaporizing species, hence they can be prepared by direct vaporization. A low-temperature sol-gel technique, including annealing at elevated temperature in oxygen if required, could also be used to obtain thin films of the oxides. 

The quantity of high-temperature data for the transplutonium oxides is very limited, which is unfortunate "as these are the actinides most like the lanthanides. Some vaporization data from a mixed PuOj g/Am203 system have indicated that the main mode of vaporization/decomposition for americium sesquioxide is via the generation of Am(g) and 02(g) (Ackermann and Chandrasekharaiah 1974). This finding has been supported by other unpublished work (Kleinschmidt 1992, Haire 1992a). The solid monoxide of Am has been considered to be barely stable toward disproportionation in the solid phase and its existence as a gaseous species is reasonable. Based on the tendency toward divalency for higher members in the series, the monoxide would be expected to become the most prevalent vapor species. 

The vaporization behavior for Cm sesquioxide has been reported (Smith and Peterson 1970). Mass-spectrometric data were not obtained in this study and the species were assigned based only on the systematics. The researchers believed these data support the congruent vaporization of Cm203 to yield CmO(g) and 0(g). An enthalpy of vaporization of 1795kJmol at OK was derived. Curium oxides with O/M ratios higher than 1.5 are not stable above 900°C (Cm02 decomposes above 400°C), and therefore the vaporization processes of oxides having O/M ratios above 1.5 cannot be measured experimentally. 

With the lanthanide sesquioxides, the high-temperature vapor species encountered above the molten or solid oxides range from R, RO, RjO and R2O2. Thermodynamic properties of the lanthanide oxides have been given in section 1 and are therefore only reviewed briefly in this section for ease of comparison. 

There appears to be two primary vaporization/decomposition modes for the lanthanide sesquioxides, which involve either the evolution of RO and oxygen, R and oxygen, or both metal bearing vapor species as shown below ... 

Vapor species over selected lanthanide sesquioxides (species formed at 2(XX)K) (Parrish 1961). 

The first actinide sesquioxide encountered is plutonium sesquioxide (if actinium sesquioxide is excluded vaporization data for it are not available) and Cf is the highest sesquioxide in the actinide series for which vaporization/decomposition data have been reported (Haire and Gibson 1992). Thus a comparison between the vaporization behaviors of the sesquioxides of the two f series can be made only between the lanthanides and the first six transneptunium oxides (Haire 1994). 

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